发布那天新闻里都提到了,不支持 iMac 2020 27 寸。
https://developer.apple.com/documentation/macos-release-notes/macos-big-sur-11-beta-release-notesRelease notes Known Issues 第一节,巨大的提醒:
General
Known Issues
Important
macOS Big Sur 11 beta 5 is not currently supported for iMac (Retina 5K, 27-inch, 2020). If you encounter an installation issue on iMac (Retina 5K, 27-inch, 2020), you need to reinstall macOS Catalina. Hold down the power button to force restart, and then press Command-R immediately after turning on your Mac. Then, follow instructions to reinstall macOS Catalina from Recovery.